Seismology

Seismology is the scientific study of earthquakes and the propagation of elastic waves through planetary bodies.

These elastic waves carry energy through both solid and fluid materials. Seismologists study these waves to understand earthquakes and their environmental effects, such as tsunamis. The field also investigates other seismic sources. These include volcanoes, plate tectonics, glaciers, and rivers. Scientists even study artificial processes like explosions. By analyzing these movements, researchers can learn about the deep structure of our planet.

When seismic energy is released, it travels as different types of waves. The first category is body waves, which move through the Earth's interior. There are two main types of body waves: P waves and S waves. P waves, or primary waves, are longitudinal waves. They involve compression and expansion, moving particles parallel to the wave's direction. Because they are the fastest, P waves always appear first on a seismogram. S waves, or secondary waves, are transverse waves. They move particles perpendicular to the direction of the wave. S waves travel more slowly than P waves. They also have low shear strength, meaning they can only travel through solids and cannot pass through fluids.

Another category of seismic energy is surface waves. These waves result from the interaction of P and S waves with the Earth's surface. There are two primary types: Rayleigh waves and Love waves. Rayleigh waves involve both compressional and shear motions. They can exist in any solid medium. Love waves are purely shear waves. They require a change in the elastic properties of a solid with depth to form. Surface waves travel more slowly than body waves because they follow indirect paths. However, their energy decays less rapidly than body waves. This means surface waves often cause the strongest shaking recorded on a seismogram.

Large earthquakes can even cause the entire Earth to ring like a resonant bell. This phenomenon is known as normal-mode motion. These normal modes consist of discrete frequencies and periods of about one hour or less. A very large earthquake can cause this ringing to be observed for up to a month. The first observations of these modes occurred in the 1960s. This happened during the 1960 Valdivia earthquake and the 1964 Alaska earthquake. These events coincided with the development of higher-fidelity instruments. Studying these modes provides strong constraints on the deep structure of the Earth.

Human interest in earthquakes stretches back to antiquity. Early thinkers like Thales of Miletus and Aristotle speculated on their causes. In 132 CE, Zhang Heng of China's Han dynasty designed the first known seismoscope. Modern science began to take shape in the 17th and 18th centuries. Athanasius Kircher suggested fire moved through channels inside the Earth. Later, John Michell determined that earthquakes originate from shifting masses of rock miles below the surface. In 1857, Robert Mallet laid the foundation for modern instrumental seismology. He is often called the "Father of Seismology" and coined the term itself.

Many scientists have used seismic data to map the Earth's hidden layers. In 1906, Richard Dixon Oldham identified different wave arrivals and found evidence of a central core. In 1909, Andrija Mohorovičić discovered the boundary between the crust and the mantle. This is called the Mohorovičić discontinuity, or the "Moho." It is defined by a change in wave velocity due to changing rock densities. In 1926, Harold Jeffreys claimed the outer core is liquid. This was supported by the fact that S waves cannot pass through it. In 1937, Inge Lehmann discovered that a solid inner core exists within that liquid outer core.

Seismology has many practical applications in the modern world. Seismometers are sensors that detect and record ground motion. A complete instrument package is called a seismograph. These tools are used in underground exploration through controlled-source seismology. Scientists use controlled explosions to map geologic traps for petroleum. They have even used seismic maps to locate the Chicxulub Crater in Central America. This massive crater was caused by an impact that contributed to the extinction of the dinosaurs.

Global networks of seismographs also serve vital safety and security roles. Because seismic waves travel much faster than tsunami waves, they allow for rapid warnings. Seismographs can also detect non-earthquake signals. These include nuclear and chemical explosions, or even large meteor strikes. One recorded strike had an energy of 4.2 × 10^13 J, equivalent to ten kilotons of TNT. Monitoring these signals is a field known as forensic seismology. It is also used to detect and study nuclear testing around the world.